(ENSO)ĕl nēnˈyō, large-scale climatic fluctuation of the tropical Pacific Ocean. The El Niño [Span.,=the child] itself is a warm surface current that usually appears around Christmas in the Pacific off Ecuador and Peru and disappears by the end of March, but every two to seven years it persists for up to 18 months or more as part of an ENSO. While the ENSO results from the dynamic and thermodynamic interactions among the atmosphere, oceans, and land surfaces, exactly what initiates an ENSO is unclear. It seems certain that pressure changes and wind currents play a vital role. Some researchers have implicated the greenhouse effect (see global warming), while others have attributed it to activity occurring on the ocean floor, such as underwater earthquakes.

In a typical ENSO, the strong easterly winds of the equatorial Pacific weaken, which allows warm eastward-flowing subsurface waters to rise, increasing surface temperatures 1–2 degrees Celsius (2–3.5 degrees Fahrenheit), and sometimes as much as 4–6 degrees Celsius (7–11 degrees Fahrenheit), in the central and E Pacific. Along the W coast of South America, El Niño's warm waters persist and deepen, and cold, upwelling, nutrient-rich waters fail to reach surface waters; the resulting warm, nutrient-poor waters devastate coastal fisheries. Heavy rain falls along the South American coast, and heavy rainfall also moves from the western to central Pacific, causing drier than normal conditions in Indonesia and nearby areas. An ENSO also affects the climate of the northern latitudes, particularly North America, which experiences warmer temperatures along the Pacific coast, increased rainfall in the Gulf states, and weaker Atlantic hurricanes.

Severe ENSO events can be economically disruptive worldwide. Of the 29 ENSOs that have occurred between 1700 and 1999, the 1982–83 El Niño was the strongest and most devastating. It caused droughts in Africa, Australia, India, Indonesia, and the Philippines, flooding in Peru and Ecuador, and devastating coastal storms in California. The ENSO was blamed for 1,300–2,000 deaths and more than $13 billion in damage to property and livelihoods.

The effects of El Niño were documented in Peru as early as the Spanish conquest in 1525. By the end of the 19th cent. the phenomenon was being studied by Peruvian oceanographers, although the effects were thought to be limited to the W coast of South America. It was not until the systematic studies of the International Geophysical Year of 1957–58 that the extent of the meteorological impact of El Niño was recognized.

La Niña, a similar climatic fluctuation, involves the abnormal cooling of the waters off Ecuador and Peru. Penetrating westward, the cold current is believed to affect weather in areas in the middle latitudes in the western Pacific Ocean and to cause extremely hot summers in Japan.

See M. H. Glantz, Currents of Change: El Niño's Impact on Climate and Society (1996); B. Fagan, Floods, Famines, and Emperors: El Niño and the Fate of Civilizations (1999).

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